Defect states in substoichiometric tantalum carbide.

Abstract

X-ray photoelectron spectroscopy was used to determine core-level binding energies (BE's) and valence-band structure for ${\mathrm{TaC}}_{\mathrm{x}}$ over the range 0.5\ensuremath{\lesssim}x\ensuremath{\lesssim}1.0. As x decreased from about 1.0 to about 0.5, the tantalum 4d and 4f BE's shifted toward the Fermi level by about 0.8 eV and the p-d valence-band peak shifted by about 0.6 eV. The carbon 2s BE was difficult to determine precisely, but it too appeared to shift toward the Fermi level as x decreased, though by a smaller amount than did the p-d valence-band peak. The carbon 1s BE shifted by yet a smaller amount, and possibly even remained constant for x>0.6. Also, the relative intensity in the valence-band spectra between about 1 and 2 eV BE increased as x decreased. These observations are explained in terms of (i) an added negative charge in the vicinity of tantalum atoms owing to the presence of carbon vacancies and (ii) the corresponding redistribution of tantalum states, i.e., the formation of occupied defect states. In addition, as x decreased it appears that charge may have transferred away from carbon atoms remaining in the crystal.